The ‘Hold it Against Me’ singer has walked off set on a number of occasions while judging auditions on her first of ‘The X Factor’ in the US, and a source says this is because she can’t take medication for her disorder.

The source told RadarOnline: “Britney was diagnosed with ADHD in her teens and used to take medication for it.

“The medication helped her deal with it, however, she can’t take it now because it interferes with the other medications she takes for her mental health issues.

“Britney’s current team of doctors strictly prohibit her from taking Adderall or Ritalin, which are commonly given to people with ADHD.”

Show head Simon Cowell – who appears alongside Britney, Demi Lovato and L.A. Reid on the new series of the show – is said to be fully aware of Britney’s condition and supports her taking breaks.

The source continued: “Simon was made fully aware of Britney’s medical history before the contract was signed, so this certainly isn’t a big surprise that she needs to take breaks during the audition process. “Britney basically has the attention span of a 10-year-old at times. It’s not alarming to Simon at all and he is fully supportive of Britney doing whatever she needs to do during filming.”

Symptoms of the disorder include overall restlessness, poor sustained attention to tasks and impaired impulse control, meaning it is hard for the ‘Piece of Me’ hitmaker to stay in one seat and concentrate on performers for any length of time.

The insider added: “Britney’s medical team works with her to deal with her disorder without medication and daily vigorous exercise is what often works best for her.

“Her nutrition is closely monitored and so is her caffeine intake, because that can also enhance Britney’s inability to focus.

“Of course, it’s rather disconcerting for people that see her do it for the first time but it’s truly nothing to be alarmed at. In fact, Simon would have been surprised if she hadn’t taken breaks during judging last week, this means she is involved and engaged in the process.”

Britney’s joining ‘The X Factor’ made headlines when she negotiated a one year contract for $15 million for her to appear.